Stranger Things 5 Finally Solves the Upside Down's 10-Year Mystery

The decade-long wait is over. Stranger Things 5 has finally pulled back the curtain on its biggest secret: the true origin of the Upside Down. And it’s not what many fans thought.

According to Entertainment Weekly, series creators Matt and Ross Duffer unpacked the long-held secret in the Season 5, Volume 2 release on December 25, 2025. They revealed that early in the show's development, Netflix prompted them to write a 20-page mythology document, laying the groundwork for a reveal they've planned for years.

Not a Dimension, But a Bridge

The truth is uncovered in the show by Dustin Henderson (Gaten Matarazzo) through Dr. Brenner's old journals. The Upside Down isn't an alternate dimension filled with monsters. Instead, it’s a wormhole—a bridge.

Years ago, when a young Eleven made psychic contact with a Demogorgon, that connection created the Upside Down as a link between our world and a much darker dimension, now named 'the Abyss'. All the terrifying creatures, including the Demogorgons, originated from the Abyss. It's also where Vecna roamed alone for years, becoming more monstrous.

From 'Dimension X' to 'The Abyss'

The Duffer brothers explained their approach was always grounded in science-fiction ideas to make the horror feel more real. Interestingly, an early name for this other dimension in the Stranger Things bible was 'Dimension X.' They scrapped it because it's a 'Teenage Mutant Ninja Turtles' reference that didn't become mainstream until after the show's 1983 setting.

"It felt more appropriate to stick to D&D references," Ross Duffer said. The name 'the Abyss' was newly conceived for this season, with Matt Duffer adding that he loved the James Cameron film of the same name.